The present invention relates generally to methods for treating a bath of molten steel to adjust the composition thereof and more particularly to a method for adjusting the dissolved oxygen content in molten steel.
Molten steel is generally prepared in a steel refining furnace such as a basic oxygen furnace, an electric furnace and, in decreasing utilization, an open hearth furnace. Molten steel prepared in a steel refining furnace generally contains dissolved oxygen which is usually regarded as an undesirable impurity. A conventional expedient for removing dissolved oxygen from molten steel is to add elements, such as aluminum, silicon, titanium or zirconium, that form stable oxides. These metal elements are referred to hereinafter as solid deoxidizing agents. A deoxidizing treatment employing a solid deoxidizing agent is usually conducted outside of the steel refining furnace, typically in a ladle into which the molten steel has been poured from the steel refining furnace.
In certain steels, sulfur is added to the steel to improve the machinability of the steel. Sulfur combines with manganese to form manganese sulfide inclusions in the solidified steel, and these inclusions improve the machinability of the steel. Manganese sulfide inclusions have a tendency to be elongated in the direction of rolling when a solidified steel casting is rolled into a shape, and elongated manganese sulfide inclusions are less desirable from a machinability standpoint than globular manganese sulfide inclusions. Likewise, smaller manganese sulfide inclusions are considered less desirable than larger inclusions.
If a certain quantity of dissolved oxygen is retained in the molten steel (e.g. 60-150 parts per million (ppm) (mg/kg)), upon solidification, the retained oxygen combines with the manganese sulfide to form oxygen containing manganese sulfide inclusions (manganese oxysulfides) which are more resistant to deformation or elongation during rolling than are those manganese sulfide inclusions formed in steel containing very little dissolved oxygen. The retained oxygen also increases the size of the inclusions. The end result of the retained oxygen is the formation of relatively large, globular manganese oxysulfides in the rolled steel shape.
Although it is desirable to retain in the molten steel a limited amount of dissolved oxygen, it is undesirable to retain in the molten steel a dissolved oxygen content above that needed to provide relatively large, globular manganese oxysulfides. However, if the surplus dissolved oxygen content is removed with solid deoxidizing agents, this forms, in the solidified steel, oxide inclusions which can have a detrimental effect on machinability. Accordingly, it is undesirable to control the surplus dissolved oxygen content in a free machining steel with solid deoxidizing agents.
In cases where the dissolved oxygen content is less than that required to provide the desired globular manganese oxysulfides, the dissolved oxygen content must be increased.
Molten steel prepared in a steel refining furnace is conventionally poured from the furnace into a ladle from which the molten steel is introduced into a casting mold which may be either an ingot mold or a continuous casting mold. If the steel is flowed into a continuous casting mold, it is first flowed from the ladle into a tundish which contains one or more outlet openings through which the steel flows to the continuous casting mold. Some tundishes contain internal structure in the form of baffles, dams, weirs and the like to control or direct the movement of the molten steel through the tundish, and this, as well as the general configuration of the tundish and its entry and exit locations, causes the molten steel to undergo a mixing action as it flows through the tundish. Embodiments of tundishes containing the internal structure and general configuration discussed above are disclosed in Jackson, et al., U.S. application Ser. No. 808,570, filed Dec. 13, 1985, and the disclosure thereof is incorporated herein by reference.
The bath of molten steel in the ladle is usually covered with a slag layer, and the molten steel in the tundish can also be covered with a slag layer. Typically, the slag layer on the molten steel in the ladle or in the tundish comprises, at least to some extent, slag from the steel refining furnace in which the molten steel was initially prepared.
In both the ladle and the tundish there is an interface between the bath of molten steel and the slag layer. In the tundish, the area of this interface per unit mass of molten steel is relatively large while in the ladle the area of this interface per unit mass of molten steel is relatively small. In the tundish it is several times greater than in the ladle.
The bath of molten steel in the ladle can be stirred by bubbling gases, such as argon, through the bath in the ladle, by electromagnetic stirring, by alloy injection, etc. As a result, there is a substantial turnover of molten steel at the interface between the bath of molten steel and the slag layer in a ladle in which the bath of molten steel undergoes stirring.
Typically, there is dissolved oxygen in the bath of molten steel, and in the covering slag layer there are oxides, such as manganese oxide (MnO) and iron oxide (FeO), having a cation corresponding to one of the metallic elements (Mn, Fe) in the bath of molten steel. The dissolved oxygen in the molten steel and the oxides in the slag layer usually move toward equilibrium with each other, i.e. the relative proportions of each move toward stable values absent some external disruption. There is movement toward equilibrium because of the natural tendency for chemical reactions to occur and to continue until they produce a state of equilibrium. The respective amounts of dissolved oxygen and slag layer oxides which are in equilibrium can be calculated from available thermodynamic data.
For a bath of molten steel made in a basic oxygen furnace and which is covered in the ladle with a layer of slag from the same furnace and to which ferro-manganese has been added at the ladle, the movement toward equilibrium is typically in the direction whereby oxygen from the slag oxides enters the molten steel to increase the dissolved oxygen content thereof. As the temperature of the molten steel bath drops, the amount of dissolved oxygen which the molten steel will hold in equilibrium also drops.
As noted above, a molten steel bath in a ladle may be stirred with an inert gas such as argon. The stirring gas may also contain, in addition to argon, carbon monoxide. For a given carbon content in the molten steel, there is an equilibrium between the carbon monoxide in the stirring gas and the carbon and oxygen in the bath of molten steel through which the carbon monoxide gas flows. The respective amounts of each which are in equilibrium can be readily calculated from available thermodynamic data.